Precision high pressure control assembly

ABSTRACT

A precision high-pressure control assembly for supercritical fluids comprises a continuous flow system having a pressure control loop which includes a source of fluid communicating with a pump. A pressure sensor monitors the pressure of the supercritical fluid in the outlet line of the pump. A pressure controller has an input for receiving a signal relating to the pressure sent by the pressure sensor and the pressure controller yields an electronic output signal to an electropneumatic regulator. A source of air communicates with the electropneumatic regulator to provide pressurized regulated driver air directed to the pump. The electropneumatic regulator controls the regulated driver air pressure of the pump in accordance with the signal received from the pressure controller.

BACKGROUND OF THE INVENTION

The present invention relates to improved assembly or system for thecontrol of a pump to increase and closely maintain the desired pressurefor a supercritical fluid.

The critical point for a substance is the temperature and pressure wherethe liquid and vapor phases exist in equilibrium. Above the criticalpoint, the supercritical range, the vapor will have the high densitiesof a liquid but the diffusion coefficient of a vapor. The vapor lookslike a "gas" but acts like a "liquid" and is called a supercriticalfluid which has the superior dissolving and extracting properties of aliquid.

Supercritical fluids may function as a superior media for chemicalreactions. Once a supercritical reaction is complete the fluid is ventedto leave just the reaction product. Supercritical fluid reactions offerenhanced reaction rates and selectivity. Additionally, supercriticalfluids technology may be used for chromagraphy and infusion as well asclose pressure control or processing within the supercritical range.

Various attempts have been made to provide controls for supercriticalfluids. For example, U.S. Pat. No. 4,684,465 relates to an apparatus foranalyzing a process stream via supercritical fluid chromatographywherein the monitoring and control functions are related to controllingthe flow rate. The '465 patent is an auto sampler which uses only onepump cycle and thus does not involve a continuous flow.

SUMMARY OF THE INVENTION

An object of this invention is to provide a system or assembly forsensing the desired supercritical fluid substance pressure and closelycontrol the process pump to maintain the desired pressure thus providingenhanced properties for a specific application.

An assembly for the successful application of this technology is theaccurate sensing of pressure, which feeds a signal to a processcontroller with an electric signal output. The output electrical signalfrom the process controller provides the input signal to anelectropneumatic regulator which is used to control the drive pressureto a positive displacement pump, for example a 100:1 diaphragm pump. Forthe CO₂ system, the process input to the pump is liquid CO₂ at 800-1,000psi, and output pressure from the pump may be up to approximately 10,000psi.

The unique assembly and programming of the downstream pump dischargepressure sensor, process controller, electronic signal controlleroutput, and electropneumatic pressure regulator control valve haveenhanced the useful application of supercritical fluid technology.

THE DRAWINGS

FIG. 1 is a schematic representation of a preferred embodiment of theinvention; and

FIG. 2 is a graph comparing the invention and prior art ramp/dwellcharacteristics.

DETAILED DESCRIPTION

FIG. 1 illustrates a preferred embodiment of this invention. As showntherein, the assembly of this invention is utilized for the handling ofCO₂. A source 10 of liquid CO₂ would be provided with the liquid CO₂ atlow pressure of about 800-1,000 psi. The CO₂ would be fed to a pump 7and discharged from the pump through outlet line 4.

Pump 7 functions to compress the CO₂ whereby the high pressure creates asupercritical fluid. Pump 7 may be of any suitable construction, aslater described, and preferably is a diaphragm driven piston pump of100:1 ratio.

The pressure of the CO₂ in outlet line 4 is sensed by a suitablepressure sensor 3 which accurately monitors the desired high pressure(critical fluid) of the CO₂.

Pressure sensor 3 provides an input signal 2 to process controller 1.Process controller 1, in turn, yields a control output signal 5 to anelectropneumatic regulator 6. Regulator 6 controls the diaphragm pump 7to compress the CO₂ fed from reservoir 10. Thus, the pump 7 transfersthe CO₂ from the reservoir 10 and elevates and maintains the desiredsupercritical fluid pressure. In this manner, liquid CO₂ in reservoir 10is fed under low pressure such as about 800-1,000 psi to pump 10 and isdischarged in outlet line 4 under high pressure of, for example,800-20,000 psi.

In operation, air such as house air at a pressure of about 100 psi isfed from a conventional compressor 9 and is closely controlled to becomethe regulated driver air 8 at pressures ranging from 0 to full 100 psipressure and then to the pump 7. The high pressure in outlet line 4 ismonitored by pressure sensor 3 to rapidly and continuously provide theinput 2 to process controller 1. The control outlet signal 5 fromprocess controller 1 sends an electronic signal back to theelectropneumatic regulator 6 and the cycle is repeated. The cycleoperates in a continuous manner and permits precise adjustments of thepressure resulting in outlet line 4.

An advantageous use of the invention would be to accurately programvarious pressure ramps and dwells within a tolerance of ±1 psi. FIG. 2,for example, compares the accuracy in achieving ramps/dwellscharacteristics with use of the invention as contrasted to conventionalprior art techniques. As shown in FIG. 2 the command pressure is plottedagainst time. The curve 11 shown in solid represents a ramp/dwell curveby practice of the invention where there could be precise pressurecontrol. Thus, as shown in FIG. 2 a ramp of uniform slope would beachieved over a set period of time until a dwell or plateau is reachedwhich is then held precisely at the desired pressure for the next periodof time. After the dwell period of time another uniform ramp would beachieved resulting in a precise dwell, etc. In contrast, curve 12reflects prior art characteristics of attempting to achieve ramps anddwells where there is not the precise control of pressure as with thepresent invention. As shown therein by the curve 12 both the ramps anddwells are irregular.

Very close pressure control is thus the key to effectively achievingprecise pressure-temperature ramping to effect solvent properties of asupercritical fluid for selective extraction of target compounds from acomplex mixture. Accordingly, this technique provides a very costeffective and environmentally friendly alternative rather than the moretraditional solvent extraction techniques for separation of mixtures.

By close control of pressure different components of a mixture may besolvated using either static extraction or dynamic extraction or acombination. By understanding the phase diagrams of the mixtures,pressure and temperature of the supercritical fluid may be controlled toselect and analyze material of interest to be removed.

In general, lower temperatures and higher pressures are desired to avoidfunctionalizing, denaturing, or thermal transition.

It is to be understood that although the invention has been describedwith respect to the handling of carbon dioxide, the invention may alsobe used for other substances, including but not limited to propane,ethane, pentane, isobutane, ammonia, nitrogen, and various otherfluorocarbons.

The following table lists the critical conditions for various solventswith which the invention could be practiced.

    ______________________________________                                                      Critical Temperature                                                                       Critical Pressure                                  Solvents      (°C.) (bar)                                              ______________________________________                                        Carbon dioxide                                                                              31.1         73.8                                               Ethane        32.2         48.8                                               Ethylene      9.3          50.4                                               Propane       96.7         42.5                                               Propylene     91.9         46.2                                               Cyclohexane   280.3        40.7                                               Isopropanol   235.2        47.6                                               Benzene       289.0        48.9                                               Toluene       318.6        41.1                                               p-Xylene      343.1        35.2                                               Chlorotrifluoromethane                                                                      28.9         39.2                                               Trichlorofluoromethane                                                                      198.1        44.1                                               Ammonia       132.5        112.8                                              Water         374.2        220.5                                              ______________________________________                                    

The invention overcomes limitations of the prior art by offering a200-fold improvement in the control of pressure in the 800 to 20,000 psirange. The control systems offered by the prior art would frequentlyresult in pump output pressure several hundred psi higher than thetarget pressure, thus yielding the application of the technology to beeither inefficient or inapplicable.

The use of the invention permits higher flow rates to the order, forexample, of 19 liters per minute in contrast to the prior art flow rateswhich are only micro liters per minute. Accordingly, the practice of theinvention permits flow rates having several orders of magnitude greaterthan with the prior art. A range of flow rate possible with the presentinvention is 0.0001-19 liters per minute.

The invention may be used in various processes. Reference is made toU.S. Pat. No. 5,269,930, the details of which are incorporated herein byreference thereto, which relates to collecting analyte in asupercritical fluid extraction process. The invention may be used, forexample, in such a process.

As previously pointed out the invention permits the precise monitoringand control of pressure in the continuous system. A typical controlrange for the pressure in outlet 4 by use of the invention is 800-20,000psi with the preferred range being 1,000 to 10,000 psi.

Any suitable equipment may be used for the components in the practice ofthis invention. For example, microprocessor 1 may be a PLC direct, DL405series PLC comprised of a DL-440 CPU (no. D4-440), a 4-Channel AnalogInput Module (D4-04AD) and a 2-Channel Analog Output Manual (D4-02AD).The pressure sensor 3 may be a Pressure Transducer which could be aSetra pressure transducer, part no. 280111-10 psig. The pressurecontroller could be an air valve/volume booster which is aProportion--Air, part no. QB1TFIE100/PSR-2. The pump 7 could be aliquid/gas booster LBGP such as a Haskel Air-Driven Liquid Pump, partno. 29376-ASF-100-D1. Suitable components such as described in U.S. Pat.No. 4,684,465 (the details of which are incorporated herein) may also beused as part of the assembly or system of this invention.

What is claimed is:
 1. A precision high-pressure control assembly forsupercritical fluids comprising a continuous flow system having apressure control loop which includes a source of fluid, said source offluid communicating with a pump for compressing the fluid and increasingthe pressure of the fluid to create a supercritical fluid, an outletline leading from said pump, a pressure sensor monitoring the pressureof the supercritical fluid in said outlet line, a process controllerhaving an input for receiving a signal relating to the pressure sensedby said pressure sensor, said process controller yielding an electroniccontrol output signal to an electropneumatic regulator, a source of aircommunicating with said electropneumatic regulator to providepressurized regulated driver air directed to said pump, and saidelectropneumatic regulator controlling the regulated driver air pressureof said pump in accordance with the signal received from said processcontroller.
 2. The assembly of claim 1 wherein said pump is a positivedisplacement diaphragm pump.
 3. The system of claim 1 wherein said fluidis carbon dioxide.
 4. The system of claim 1 wherein said fluid isselected from the group consisting of propane, ethane, pentane,isobutane, ammonia, nitrogen and fluorocarbons.
 5. A method for closepressure control of a supercritical fluid comprising feeding a fluidunder low pressure to a pump, increasing the pressure of the fluid bythe pump wherein the fluid is discharged to an outlet line as asupercritical fluid under high pressure, sensing the pressure of thesupercritical fluid in the outlet line by a pressure sensor whichprovides an input signal to a process controller with the input signalbeing reflective of the amount of pressure being sensed, sending anelectronic control output signal from the process controller to anelectropneumatic pressure regulator, supplying air from an air source tothe electropneumatic pressure regulator to provide pressurized regulateddriver air directed to the pump, and controlling the regulated driverair pressure of the pump by the electropneumatic pressure regulator inaccordance with the signal received by the electropneumatic pressureregulator from the process controller.
 6. The method of claim 5 whereinthe fluid is carbon dioxide.
 7. A method for close pressure control of asupercritical fluid comprising feeding a fluid under low pressure to apump, increasing the pressure of the fluid by the pump wherein the fluidis discharged to an outlet line as a supercritical fluid under highpressure, sensing the pressure of the supercritical fluid in the outletline by a pressure sensor which provides an input signal to a processcontroller with the input signal being reflective of the amount ofpressure being sensed, sending an electronic control output signal fromthe process controller to a pressure regulator, controlling the drivepressure of the pump by the pressure regulator in accordance with thesignal received by the pressure regulator from the process controller,and wherein the fluid is selected from the group consisting of propane,ethane, pentane, isobutane, ammonia, nitrogen and fluorocarbons.
 8. Amethod for close pressure control of a supercritical fluid comprisingfeeding a fluid under low pressure to a pump, increasing the pressure ofthe fluid by the pump wherein the fluid is discharged to an outlet lineas a supercritical fluid under high pressure, sensing the pressure ofthe supercritical fluid in the outlet line by a pressure sensor whichprovides an input signal to a process controller with the input signalbeing reflective of the amount of pressure being sensed, sending anelectronic control output signal from the process controller to apressure regulator, controlling the drive pressure of the pump by thepressure regulator in accordance with the signal received by thepressure regulator from the process controller, and wherein the pressurein the outlet line is in the range of 800-200,000 psi.
 9. A method forclose pressure control of a supercritical fluid comprising feeding afluid under low pressure to a pump, increasing the pressure of the fluidby the pump wherein the fluid is discharged to an outlet line as asupercritical fluid under high pressure, sensing the pressure of thesupercritical fluid in the outlet line by a pressure sensor whichprovides an input signal to a process controller with the input signalbeing reflective of the amount of pressure being sensed, sending anelectronic control output signal from the process controller to apressure regulator, controlling the drive pressure of the pump by thepressure regulator in accordance with the signal received by thepressure regulator from the process controller, and wherein the pressureis controlled to accurate obtain at least one ramp and one dwell of highprecision.
 10. The method of claim 9 wherein a plurality of ramps anddwells are obtained.
 11. The method of claim 9 wherein the pressure iscontrolled to an accuracy of ±1 psi.
 12. The method of claim 10 whereinthe method is used as pressure-temperature ramping to effect solventproperties of the supercritical fluid for selective extraction of targetcompounds.
 13. A method for close pressure control of a supercriticalfluid comprising feeding a fluid under low pressure to a pump,increasing the pressure of the fluid by the pump wherein the fluid isdischarged to an outlet line as a supercritical fluid under highpressure, sensing the pressure of the supercritical fluid in the outletline by a pressure sensor which provides an input signal to a processcontroller with the input signal being reflective of the amount ofpressure being sensed, sending an electronic control output signal fromthe process controller to a pressure regulator, controlling the drivepressure of the pump by the pressure regulator in accordance with thesignal received by the pressure regulator from the process controller,and wherein the fluid flows from the pump at a flow rate of from 0.0001to 19.0 liters per minute.